LIB and MP-BGP?

[parisdooz12]

Hi,

I am trying to understand interactions between different tables and protocols used in MPLS.

If I think to have well understood how LDP feed LIB table, then how the LIB feed the LFIB, it's not so clear for MP-BGP.

I have 3 questions:

1)  I have readen the following thing in a Cisco book:

"The LIB is fed by the label bindings received by LDP, Resource Reservation Protocol (RSVP), MP-BGP, or statically assigned label bindings."

Is there a LIB fed by MP-BGP? In this case, what's the command to read it? (like "sh mpls ldp binding"). If it does exist, is it a table common with the label assigned by LDP? Is it dedicated to the VRF?

2) Is there a single LFIB for a physical router (not one per VRF)? I guess so as if I understand well, the LFIB is readen to know the VRF and interface to CE, but ... not sure

3) I have readen that the label assigned by MP-BGP indicate the interface to CE more than the VRF itself. Does it mean that if the PE has 2 interfaces with 2 CEs, in the same VRF, then 2 labels are created? (one for prefixes of CE1 and another one for prefixes for CE2?)

Thanks by advance

P.

[Luc De Ghein · Accepted solution]

Hi,

1)  I have readen the following thing in a Cisco book:

"The LIB is fed by the label bindings received by LDP, Resource Reservation Protocol (RSVP), MP-BGP, or statically assigned label bindings."

Is  there a LIB fed by MP-BGP? In this case, what's the command to read it?  (like "sh mpls ldp binding"). If it does exist, is it a table common  with the label assigned by LDP? Is it dedicated to the VRF?

> LIB is fed by LDP. The label bindings from RSVP-TE and MBGP are stored by themselves. So, the labels associated with BGP prefixes are stored in the BGP tables.

2) Is there a single LFIB for a physical router (not one per VRF)? I  guess so as if I understand well, the LFIB is readen to know the VRF and  interface to CE, but ... not sure

> There is a single LFIB per router. It is not per VRF. The label lookup in the LFIB can actually determine which VRF the packet belongs to, but first the label lookup needs to be done to know that.

3)  I have readen that the label assigned by MP-BGP indicate the interface  to CE more than the VRF itself. Does it mean that if the PE has 2  interfaces with 2 CEs, in the same VRF, then 2 labels are created? (one  for prefixes of CE1 and another one for prefixes for CE2?)

> One VRF prefix will get one (local) label assigned. So, on a PE with load balancing of one VRF prefix to 2 CE routers, there will still be only one local label assigned to that one VRF prefix. The load balancing of the packets towards the 2 CE routers can still occur, because there will be 2 next hops/outgoing interfaces in the LFIB for this prefix.

Thanks,

Luc

[parisdooz12]

Hi Luc,

thanks for your answer, it's more clear now.

About question #3: what I mean is a case where a PE is attached to 2 CE (CE1 & CE2), but each CE announcing different prefixes (so no load-balancing). What happens in this case: is there 1 label announced for all prefixes known in the VRF or 1 label for each CE? (or even more label?)

Regards

P.

[Ivan Krimmel · Accepted solution]

Hi there,

> What happens in this case: is there 1 label announced for all prefixes known in the VRF or 1 label for each CE? (or even more label?)

it depends on the label allocation mode, depending on the platform we've got per-prefix(most common and default one), per-VRF and per-CE modes.

> Even more suprrising for me, I have readed: "For MPLS VPNs, each prefix in each VRF is an FEC"

just imagine that some set of prefixes have got a different QoS policy resulting in different EXP bits turned on - here you go with different treatment of those prefixes on an MPLS core.

HTH,

Ivan.

[parisdooz12]

Ok, it's more clear, thanks.

I found details about per-VRF label here:

http://www.cisco.com/en/US/docs/ios/mpls/configuration/guide/mp_vpn_per_vrf_lbl.pdf

[Mohamed Sobair]

Hello,

Question (1): Yes, the LIB is fed by LDP and MP-BGP is used to distribute labels and for LDP Signaling as described by Compella Draft.

There are Two Draft: 1- Compella Draft . 2- Martini Draft

Compella uses MP-BGP for LDP Signaling while Martini uses Label Distribution Protocol LDP for signaling.

The Command in Cisco to view the KDP binding is (Show mpls ldp binding).

Question (2):  We need to distinguish between Control plane and Data Plane:

                      The LFIB is Data Plane Table and its table created only for  Label Forwarding in the Core of the MPLS Network. and this table is created based on the LIB.

                      The LIB is a Control Plane table and its a table created for each Forward Equivelan Class, the FEC is a set of layer-3 and Layer-2 information.

Question (3):  The Label is not assigned to VRF, the Label is assigned to each seperate FEC , So if you have multiple interfaces on the Same VRF, it should have different label bindings indeed.

Hope this Helps,

Mohamed

[parisdooz12]

Hi Mohamed,

thanks for answer.

About question #3, the FEC is not clear for me in case of VRF, if I take the following FEC definition: "

FEC is a set of packets that receives the same forwarding treatment by a single LSR". For me, all prefixes destinated to a VRF ahev the same treatment as soon as they arrive on the PE (from MPLS).

Even more suprrising for me, I have readed: "For MPLS VPNs, each prefix in each VRF is an FEC" (

http://www.ciscopress.com/articles/article.asp?p=1081501&seqNum=5) ??

Regards,

P.